The present invention is directed to telephone-call-processing systems and in particular to systems for handling calls on time-division-multiplexed channels such as those implemented on Integrated Services Digital Networks ("ISDNs").
Many installations use computers to control telephone traffic into and out of those installations. In doing so, they operate call-processing apparatus, which responds to instructions from the computer by performing the lower-level operations required for call processing. Such apparatus also reports to the computer concerning the message traffic that it handles.
For instance, if the computer is to implement call forwarding, the call-processing circuitry would interpret the dial signals and report the numbers thus dialed to the computer. The computer would determine whether the call should be connected to the on-site equipment designated by that called number or forwarded to, say, off-site equipment associated with a different number. The computer would then direct the call-processing apparatus accordingly, causing it, say, to dial up the station to which the call is to be forwarded, detect the resultant ringback signals, and send ringback signals in turn to the originating station.
This, of course, is merely one example of the many types of functions that a call-processing equipment might perform, and it is often desirable that the same call-processing apparatus be adaptable to perform several such functions. For this reason, the call-processing equipment might be arranged to respond to considerable low-level control by the computer; i.e., the computer's programing could be used to provide the desired flexibility. This is impractical in many situations, however, because performance of many low-level tasks diverts the computer's resources from other functions, such as, say, record keeping and billing, that the computer may be required to do.
In the case of some call-processing equipment employed for conventional, "analog" channels, flexibility has been afforded without requiring extensive low-level operation by the host computer. Such equipment provides the call-processing system's operator with a keyboard or other manual input device by which he can enter behavior specifications into the call-processing apparatus. Alternatively, the behavior specifications can be entered by, e.g., computer. In any event, the specifications are stored for use in real time so that the host computer does not itself need to control the details of the processor's behavior in real time.
Specifically, a command that an operator might enter by way of the keyboard may be one that specifies whether the response to a call to be forwarded is immediate ringback, on the one hand, or, on the other hand, ringback only after the call-processing equipment itself receives a ringback signal from the forward-to station. Such equipment thus provides the desired flexibility without overly burdening its host computer.
A number of call-processing systems, however, are required to operate with ISDN networks, which do not then send normal supervisory (off-hook, ringback, etc.) signals that are the subjects of the analog-call-system keyboard commands of the types described above. Such networks convey a plurality of calls on a single two- or four-wire trunk line by time-division multiplexing. Since a given trunk line is not typically dedicated to a single call at any one time, conventional supervisory signals are inadequate to control the call traffic. The approach described above to achieve call-processing flexibility for analog networks thus is not effective for call processing on ISDNs.